Car bodies are mainly assembled by joining press-formed steel sheets by resistance spot welding. In the resistance spot welding used in the assembly of car bodies, both securing a nugget size corresponding to the sheet thickness and suppressing the occurrence of spatter are sought.
In general, for example, a range defined by a current value giving a 4√t (“t” shows the sheet thickness (mm)) or other reference nugget size as a lower limit (below, referred to as the “lower limit current” or “4√t current”) and a current value where spatter occurs as an upper limit (below, referred to as “upper limit current” or “spatter current”) (below, referred to as a “suitable current range”) is considered an important indicator relating to spot welding of steel sheets. The lower limit current and the upper limit current are measured in the ideal state at the test piece.
Spatter includes inner spatter (the phenomenon of metal of the base material melted by the welding scattering from the superposed surfaces of the steel sheets) and outer spatter (the phenomenon of the base material metal melted by the welding scattering from the contact surfaces of the steel sheets and electrodes). In both cases, the surface quality is lowered by scattering and sticking to the car body. Further, sticking to the moving parts of a welding robot causes poor operation of the equipment. Furthermore, the outer spatter remaining in needle shapes on the surface of a spot welded part becomes a cause of damage to the wire harnesses of automobiles etc., so a grinder has to be used for grinding away the spatter. For this reason, it is required that in resistance spot welding, inner spatter and outer spatter be avoided and a predetermined nugget size be secured.
The lower limit current is evaluated in the ideal state at the test piece level. However, in actual assembly of a car body, due to wear of the electrodes, shunting to existing welding points, clearance between pressed parts, and other various external factors, even if welding an actual car body by a current value giving 4√t at the test piece level, the nugget size will sometimes fall below 4√t. For this reason, on a mass production line, it is necessary to set a current value of 1.0 kA or more, preferably 1.5 kA or more, higher than the current giving 4√t at the test piece level as a realistic lower limit current value. Therefore, when desiring to stably obtain a 4√t or more nugget size on a mass production line without spatter occurring, the suitable current range in evaluation at the test piece level is required to be 1.0 kA or more, preferably 1.5 kA or more. This is because if a predetermined suitable current range cannot be secured at the test piece level, it is necessary to set the current value at a current where spatter occurs to stably secure a 4√t nugget size in spot welding at the actual worksite where there are many outside disturbances.
In recent years, in assembly of automobiles, resistance spot welding machines of the inverter DC type instead of the single-phase AC type have increasingly been used. The inverter DC type enables the transformer to be made smaller, so there is the merit of enabling a robot with a small carrying load to carry it, so this is particularly often used on automated lines.
The inverter DC type does not turn the current on and off like the conventionally used single-phase AC system but continuously applies current, so the heat generation efficiency is good. For this reason, it is reported that even in the case of a zinc-plated material of thin sheet soft steel where formation of a nugget is difficult, a nugget is formed from a low current and the suitable current range is broader than the single-phase AC system.
On the other hand, if welding high tensile steel sheet, where formation of a nugget is easy, by an inverter DC power supply, conversely to a soft steel sheet, the current where spatter occurs is low. That is, sometimes the upper limit current becomes lower and therefore the suitable current range becomes remarkably narrower. In resistance spot welding, as shown in FIG. 1, a single-stage conduction system performing conduction just a single time is often used in resistance spot welding of cars. However, with the single-stage conduction system, the suitable current range becomes narrower, so a conduction system which broadens the suitable current range has been reported.
PLT 1, as shown in FIG. 2, discloses a method employing a two-stage conduction system using preliminary conduction to improve the fit between contact surfaces of the steel sheets, then perform the main conduction so as to suppress the occurrence of spatter in resistance spot welding of high tensile steel sheets.
PLT 2, as shown in FIG. 3, discloses a method employing a conduction system using preliminary conduction to improve the fit between contact surfaces of the steel sheets, then stopping the current, then using main conduction so as to suppress the occurrence of spatter in resistance spot welding of high tensile steel sheets.
PLT 3, as shown in FIGS. 4 and 5, has a three-stage conduction process. That is, it is comprised of a first step of forming a nugget as a preliminary conduction step, a second step of making the current fall after the preliminary conduction and increasing a corona bond area around the nugget, and a third step of running a current larger than the preliminary conduction current after the second step and enlarging the nugget size as a main conduction step. In this method, due to the preliminary conduction, the fit between the contact surfaces of the steel sheets is improved, then the current is lowered, then main conduction of a certain current or main conduction of a pulsation shape is performed so the occurrence of spatter at the resistance spot welding of the high tensile steel sheet is suppressed. Furthermore, PLT 3 discloses making the third step a pulsation conduction system to thereby make the effect of expansion of the conduction diameter greater and suppress the occurrence of spatter compared with the continuous conduction system.
PLT 4, as shown in FIG. 6, discloses a method of using resistance spot welding repeating an up-down change of current while raising the current value to suppress the occurrence of spatter in resistance spot welding of high tensile steel sheets.
NPLT 2, as shown in FIG. 7, discloses a resistance spot welding method for thickness 1.5 mm or more steel sheets which comprises conduction for 120 msec (six cycles at 50 Hz) or more and idling for 40 msec (two cycles at 50 Hz) repeated three times or more.